We've seen Kuma's mechanics withstand a pre-skip Luffy's Jet Pistol. A post-skip Jet Pistol with the invisible variant of Haki destroyed the entire mechanical structure of a Pacifista via a blow to the head. The mechanical structure of a Pacifista, at least the model we have seen so far of which Kuma is the same, cannot withstand a blow from current Luffy while he is using Gear 2. Compare that to Gladius, who took a Jet Stamp to the gut and was fine.
Kuma reacted to a pre-skip Luffy, who is exponentially weaker and slower than Doflamingo. Kuma reacting to and countering pre-skip Luffy does not mean he can counter Doflamingo.
Kuma called himself a Pacifista while in control. Prior to Marineford, Kuma wasn't a complete Pacifista only because he retained his consciousness. Every other modification was complete, meaning his mechanical structure was the same as the other Pacifistas at the time he was in control.
You have no idea that ordinary people were used, or that none of the Pacifista didn't have bodies like Kuma's. You also have no idea how strong Kuma's body was prior to the modifications to say that he was special or abnormally more durable in comparison to the others who were made into Pacifista. Even if Kuma's body was stronger, the mechanics are still the same. The mechanics are what Luffy broke and what exploded. Even though Kuma's body may be level 50 while the others were level 30, the mechanics are still the same level because they are the same modifications; a level not strong enough to withstand a simple Jet Pistol from Luffy with the invisible Haki equipped.